1. Field of the Invention
This invention relates to the continuous testing of strip material for determining the magnetic characteristics thereof.
More particularly, the invention is concerned with apparatus which uses a non-contacting coil arrangement and associated electronic circuitry to monitor the relative magnetic quality of a moving strip of material with no line restrictions or interference. The invention is also concerned with a method for obtaining a profile indicating quality of a coil of strip material throughout the length thereof.
When a change takes place in magnetic properties, this usually indicates that a change has occurred in one of the other variables, such as steel chemistry , mechanical hardness, gauge, grain size or structure, stress relief of work-hardened strip, uniformity and degree of annealing, the presence of faults or occlusions or the like. Such changes are not observable by eye and are generally difficult to detect. A large quantity of defective steel strip may be produced, or stated another way, the steel strip which is produced may not be of uniform quality throughout, and while it may be suitable for one purpose, it may not be suitable for its primary purpose.
It has also been observed that flux will travel further from the source in a good quality strip than in one of poorer quality.
2. Description of the Prior Art
Numerous systems have been developed over the years and several systems are in current use, both in the United States and elsewhere. Most of these use an open solenoid, a single, or a double yoke approach. The magnetizing force is indicated by means of an "H" indicating device such as Chattock coil, Hall probe or exciting current. Induction in the strip is indicated by voltage produced in a coil encircling the strip. Electronics are employed to indicate core loss, permeability or a number correlated to magnetic quality.
For example, U.S. Pat. No. 3,130,363 is concerned with apparatus for determining the magnetic condition of a moving strip, and in particular, the magnetic condition of steel strip being continuously annealed in order to adjust for proper annealing temperature. In particular, use is made of a detector head mounted in a tube which is placed into a heating chamber of a furnace to ascertain whether the temperature of the strip is above or below the Curie point. The detector head used is a single H-shaped core having identical coils wound on the legs. Operation is based on ascertaining when the system is below the Curie point, and at that time the detector head indicates an imbalance. When balanced, there is no conductivity.
U.S. Pat. No. 3,281,678 is also concerned with determining the magnetic properties of strip material. It is known that the quality of sheet or strip steel is dependent upon its magnetic properties. For this purpose, this patent is directed to a magnetic core loss tester and uses only two coils. One is a "B" coil and the other is an "H" coil which provides an indication of core loss in watts per pound. A ratio meter is also used to obtain magnetic permeability or .mu. and this reads the ratio of B/H directly. Accordingly, only two coils are used which surround the strip steel, the magnetizing coil B and the sensing coil H. In effect, this patent discloses sophisticated circuitry for obtaining core loss and permeability.
U.S. Pat. No. 3,421,925 is also concerned with the improvement of the final product of strip steel as well as to avoid relatively poor electrical performance of strip steel. For this purpose, this patent proposes method and apparatus to continuously test the strip for core loss or watts loss per pound as it is being processed. The test apparatus includes a conventional "M" coil for applying a magnetizing force of predetermined magnitude, a "B" coil for measuring the total flux in the strip and the surrounding air, and an "H" coil for measuring the flux in the surrounding air only. This patent must use the thickness of the steel as one of its variables, and for this purpose, it employs an X-ray gauge to determine the thickness of the steel, and the signal from the "B" and the "H" coils are coupled with the signals from the X-ray gauge to produce a continuous record of watts loss per pound of the material, and thereby provide a record of the electrical quality of the steel.
U.S. Pat. No. 3,444,458 is also concerned with the detection of variations in the magnetic properties of steel, and more particularly, for detecting variations in the quality of continuous steel strip moving in a processing line. This patent discloses a detecting device which includes a pair of matched cores for placement on opposite sides of a strip of ferromagnetic material with their ends in opposition and provided with a space or air gap through which a strip to be tested is passed. The cores are energized and by passing the strip material through the air gap, the reluctance of the circuit is changed. When there is a change in the output voltage, this indicates a change in the characteristics of the strip. Use is made of two windings on a single core, but one winding is used as a reference winding to obtain a zero or null position on an output meter and the other winding is used as an excitation winding.
Siemens German Pat. No. 1,120,591 is also concerned with the measurement of the magnetic properties of electrical sheet strip which may be of non-uniform cross-section. When the cross-section is not uniform, then, as is well-known, variations due to magnetic inductions occur so that the measurements taken are not exact. This patent proposes the use of rollers positioned at the yoke ends which can be raised to provide a voltage proportional to the thickness of the material which passes under the rollers. A number of coils are arranged in parallel, but are aligned in a tandem relationship as the strip passes through the coils so that only the strip is magnetically energized. Since the rollers are vertically movable, correction of the magnetic values in accordance with thickness is obtained.
U.S. Pat. No. 3,748,575 is also concerned with the testing of the characteristics of a moving metal strip. A magnetic monitoring device is located within a hollow member so that any irregularities in the advancing strip do not affect the monitoring device. The magnetic characteristic of the strip is monitored by using a first magnetic head to record a magnetic signal onto the strip and then a second magnetic head displaced from the first magnetic head is used to sense the signal so recorded as the strip passes the second magnetic head. The two magnetic heads are required to be maintained at a fixed separation and are angularly spaced from each other by a fixed predetermined angle. A magnetic screen is also interposed between the two magnetic heads to prevent interaction therebetween. Regular, but intermittent and non-continuous readings of the magnetic characteristics are obtained.
British Pat. No. 928,500 is also concerned with method and apparatus for the measurement of magnetic properties, such as core loss and permeability of a moving strip of steel. A magnetic yoke is brought into contact intermittently with a traveling steel strip which yoke has an excitation and an induction winding. Current is induced into the strip and the induced current is measured to provide an induction of the magnetic, and electrical, properties of the steel.
U.S. Pat. No. 3,723,859 is also concerned with the testing of the characteristics of a moving metal strip and in particular with the monitoring and recording of steel characteristics in a continuously moving strip of steel. However, in this system, thickness is one quantity which is determined as well as permeability and loss of a moving strip of electrical core steel. Measurements are taken at different positions. In the system according to this patent, apparatus is provided to establish the same flux density at two different positions of the moving strip of steel.
At present, standard magnetic quality tests are conducted by removing a test panel or panels from ends of a coil. The coil is graded by core loss using these tests. If a coil end is of poor quality, it is a general practice to remove a portion of the material and to retest. The "cutback" procedure is an estimate based on experience and can be a waste of time, manpower and equipment and may result in scrapping salable material. The standard tests provide only a test of the material where the panel is removed and do not indicate magnetic quality throughout the coil. Commercial test units have been developed which are very expensive ($200,000 to $500,000) but strip width, gauge and quality variations complicate the electronics and, in general, the units may have variations in test level accuracy that are caused by changes in the quality level of the material under test.